When a rotary machine such as a motor or a generator suddenly breaks down, unplanned repair work or replacement of the rotating machine is needed, and a drop in operating ratio of the production equipment or a reconsideration of the production plan is necessary. Although it is possible to understand the extent of degradation and prevent a sudden breakdown by appropriately stopping the motor and diagnosing offline, this will result in a drop in the operation ratio of the production equipment as the rotating machine has to be stopped for an offline diagnosis. Thus, there is a growing need for on-line diagnosis which makes diagnosis possible during operation of the rotating machine. An inverter surge occurring when the rotating machine is driven at variable speeds by the inverter is one of the main causes of sudden breakdowns. Problems such as an acceleration of insulation degradation of the rotating machine resulted from the inverter surge, and an acceleration of bearing deterioration due to electrochemical corrosion of the bearing have occurred. This trend is expected to become more prominent with the advent of semiconductor devices using new materials such as SiC (silicon carbide).
Here, as a related art, for example, there is one disclosed in Patent Literature 1. In Patent Literature 1, degradation can be detected by using an optical fiber current sensor to constantly monitor the current flowing through the winding wires inside a rotating machine during operation. As a method of measuring the leakage current from the difference between the values of current flowing through the coils at two positions spatially separated from each other, it discloses a method of obtaining by signal processing and the like using two optical fiber current sensors, and a method of winding the optical fiber current sensors at the two coil positions in opposite directions from each other and with the same number of times of windings.